In the production of oil wells, well fluid may be recovered by the use of a free plunger, sometimes called a gas lift plunger or piston. This type of plunger is freely movable in a string of tubing in the well and travels between the top and bottom of the tubing. The pressure of the gas from the producing formation causes upward movement of the plunger. A slug of liquid from the oil bearing formation which has seeped into the tubing above the plunger is lifted by the plunger to an output flow line at the surface.
The cycling of the plunger is typically controlled by the opening and closing of a motor valve located in the output flow line. With the plunger at the bottom of the tubing resting against an abutment or seating nipple, and with the motor valve closed, formation gas pressure will build up over a period of time. A timing mechanism opens the motor valve after a predetermined time lapse. This establishes a pressure differential across the plunger, and greater pressure beneath the plunger drives the plunger upwardly through the tubing. Upward movement of the plunger forces oil in the tubing above the plunger outwardly through the output flow line. When the plunger reaches the top of the tubing, the motor valve is closed. Pressure across the plunger then equalizes, and the plunger falls by gravity to the bottom of the tubing. The cyclic process then starts over again.
Various types of free plungers have been used. One type of plunger is provided with a passageway therethrough which is opened and closed by a valve. During upward movement of the plunger, the valve is closed so that the interior of the tubing above the plunger is substantially sealed from the interior of the tubing below the plunger. This maintains the gas pressure differential necessary for lifting. During downward movement of the plunger, the valve is open to permit well fluid to flow substantially freely through the passageway.
Another valve-type plunger includes a circumferential, radially expandable section which is expanded (valve closed) into contact with the well tubing during upward movement of the plunger, and is retracted (valve open) during downward movement of the plunger.
While a valve in the plunger is desirable to permit faster descent of the plunger, such valves render the plunger more complex and costly to manufacture. Reliability and ruggedness is also a problem because of the moving parts involved.
Another type of free plunger is the valveless type. Valveless free plungers are typically used in low production wells where it is not necessary to quickly return the plunger to the bottom of the tubing. In a valveless free plunger system, the rate of descent of the plunger is slower because fluid beneath the plunger must flow through the small annular gap between the outer periphery of the plunger and the interior sidewall of the tubing. This annular clearance gap is the same for both ascent and descent. The gap does not widen during descent as in some valve-type plungers, nor is there a bypass passageway during descent as in other valve-type plungers.
Valveless free plungers present special and conflicting problems, particularly in the dimension of the annular clearance gap. There should be a sufficiently tight fit of the plunger within the tubing to afford a sufficiently effective seal during ascent. Yet the gap must be wide enough to allow descent at a rate which is not too slow to be practical. Too loose a fit sacrifices lifting efficiency during ascent; too tight a fit sacrifices descent rate. There is a need for a valveless free plunger which affords enhanced lift capability, yet descends at a practical rate. There is further a need for a valveless free plunger which is simple and economical to manufacture and affords accurate tolerance control.
Another problem encountered is maintenance of sealing tolerances over extended periods of use. The downhole well environment encountered by the plunger together with the close sealing tolerances dictate that the plunger be resistant to the atmosphere of the well and to frictional wear against the interior sidewall of the well tubing. On the other hand, the plunger should not be so heavy and bulky that too much of the lifting force generated by the gas pressure differential is needed just to overcome the weight of the plunger. A need has thus arisen for a valveless free plunger which is lightweight yet durable and wear-resistant to maintain sealing tolerances over an extended life rating.
There is further a need for a plunger of simple yet rugged design and construction. Various prior systems have employed elaborate cushioning or shock-absorbing apparatus at the top and/or bottom of the tubing to protect the plunger upon impact. There is a need to eliminate such auxiliary apparatus by providing a plunger which is virtually indestructible, but yet not so heavy and bulky as to sacrifice lifting efficiency, nor so complex in design and construction as to render it too costly to manufacture.
Another problem is that of making optimum use of the formation gas pressure in generating plunger lift. A need has arisen for a valveless free plunger which is not only lightweight and wear-resistant, but which also makes effective use of gas pressure lift.
There is further a need for a simplified overall operating system of the valveless free plunger type. Pumping systems with auxiliary valving and control apparatus at the surface and/or downhole are complex and costly. There is a need to provide a simple system with a minimum of parts.